well, if you are talking about unit the most powerfull is HallowenGirl, she can be used in any kind of deck (event card), and when she attacks do can made ap=4 or 0 but if attacks with ap 0 she doesn't get conter attack because her skill (charmer).

I f you are talking about spells i thing Noble Sacrifice (light) and Magic Coffing (metal) are strong. Both are anti-spell and rare cads, but really good to have in your deck.